PHYSICS OF THE EIGHTEENTH CENTURY. 297 



FIG. 144. JAMES WATT. 



water will rise ; but at the same time the pressure, tension, or elastic 

 force of the steam within the vessel will increase. At 212 this ten- 

 sion is able to balance the pressure of the atmosphere, that is, it is 

 equal to about 15 Ibs. on every square inch of surface. Hence if a 

 closed vessel containing water is heated to 212, every square inch 

 of its internal surface is pressed outward with this force. But if the 

 temperature be raised to 250 F., the pressure becomes 30 Ibs. per 

 square inch. At 300 F. the pressure is about 67 Ibs; per square 

 inch; at 358 it is 150 Ibs. ; and so on. Papin contrived the appa- 

 ratus represented in Fig. 145 for the purpose of heating water to a 

 higher temperature than 212 F. It consists of a strong iron vessel o, 

 provided with a lid, which can be forcibly fastened down by the 

 screw shown in the figure. The object of raising the temperature of 

 the water is to increase its solvent powers for culinary or other pur- 

 poses. To prevent the vessel from exploding by undue tension of 

 the steam within, Papin contrived the Safety - Valve. This is simply a 

 metallic plug, s, which closes an opening in the lid of the vessel, and 

 could be readily lifted up from within, were it not pressed down by 

 the lever L, upon which the weight p acts. When the tension of the 



